1. Field of the Invention
The present invention relates generally to ink-jet printing and, more specifically, to vacuum belt-type ink-jet printers and the utilization of multiple belts and associated devices for heating and pressing print media.
2. Description of Related Art
The art of ink-jet technology is relatively well developed. Commercial products such as computer printers, graphics plotters, copiers, and facsimile machines employ ink-jet technology for producing hard copy. The basics of this technology are disclosed, for example, in various articles in the Hewlett-Packard Journal, Vol. 36, No. 5 (May 1985), Vol. 39, No. 4 (August 1988), Vol. 39, No. 5(October 1988), Vol. 43, No. 4 (August 1992), Vol. 43, No. 6 (December 1992) and Vol. 45, No.1 (February 1994) editions. Ink-jet devices are also described by W. J. Lloyd and H. T. Taub in Output Hardcopy [sic ] Devices, chapter 13 (Ed. R. C. Durbeck and S. Sherr, Academic Press, San Diego, 1988). [For convenience of describing ink-jet technology and the present invention, all types of print media are referred to simply as “paper,” all compositions of colorants are referred to simply as “ink,” and all types of hard copy apparatus are referred to simply as a “printer” No limitation on the scope of invention is intended nor should any be implied.]
FIG. 1 (PRIOR ART) depicts a generic, vacuum belt print media transport, ink-jet hard copy apparatus, in this exemplary embodiment a computer peripheral, ink-jet printer 10. An ink-jet writing instrument 12 (also referred to hereinafter as simply a “pen”) is provided with a printhead 14 having drop generators (not seen in this view), including nozzles for ejecting ink droplets onto an adjacently positioned print medium, e.g., a sheet of paper 16, in the apparatus' printing-zone 34. An endless-loop belt 32 is one type of known manner printing-zone input-output paper transport. A motor 33 having a drive shaft 30 is used to drive a gear train 35 coupled to a belt pulley 38 mounted on a fixed axle 39; a known manner position tracking device 41 can be provided. A biased idler wheel 40 provides appropriate tensioning of the belt 32. The belt rides over a platen 36 in the print-zone 34. The platen 36 is associated with a known manner vacuum induction system 37. The paper sheet 16 is picked from an input supply (not shown) and its leading edge 54 is delivered to a guide 50, 52 where a pinch wheel 42 in contact with the belt 32 —or the belt vacuum force itself —grips the leading edge of the sheet to continue transport of the paper sheet 16 through the printing-zone 34 (the paper path is represented by arrow 31). Downstream of the printing-zone 34, an output roller 44 in contact with the belt 32 receives the leading edge 54 of the paper sheet 16 and continues the paper transport until the trailing edge 55 of the now printed page is released; in some implementations, suction force release is sufficient for allowing the sheet to leave the printing-zone 34 transport mechanisms. A system controller 62 provides the necessary signals for paper transport, writing instrument 12 operations, and the like as necessary for printer 10 operations. The carriage scanning axis is conventionally designated the x-axis, the print media transit axis is designated the y-axis, and the printhead firing direction is designated the z-axis.
One source of image quality degradation is print head crashes on the media surface. These crashes can be induced by the media rising up off the main printing belt into the swept volume of the printheads. The cause of the media buckling is usually due to the wet colorant ink-jet printing process itself. As the fluid from the ink droplets is absorbed by the paper fibers, regions of the media expand differently as a function of the volume of ink in the region. This is also referred to as “cockle,” an irregular rather than planar surface produced in paper by the saturation and drying of ink deposits on the fibrous medium. As a sheet of paper gets saturated with ink, the paper grows and buckles in a seemingly random manner. Paper printed with images are more saturated with colorant than simple text pages and thus exhibit great paper cockle effects. Colors formed by mixing combinations of other color ink drops form greater localized saturation areas and also exhibit greater cockle tendencies.
One known solution for this problem is using a combination of heat, vacuum, and airflow to dry the media quickly, holding it down during the critical time just after ink deposition. However, this drying of the ink can also cause problems in local environmental conditions. Moreover, when media sits in a high humidity environment, it absorbs water from the air and stores the moisture in its fibrous structure, causing expansion. Therefore, even pre-printing, paper moisture content is a significant problem. Under common ambient atmospheric conditions (e.g., an office environment having a relative humidity of about 80% at 30° C.), paper commonly used for ink-jet printing can have a water content that is significant to the process. Depending on actual humidity, the moisture content of paper can be from about 1% to 10%. If an expanded sheet is then brought into a high temperature location, such as a heated print zone, the moisture in the fibers will be driven out and the media again will try to shrink. If this shrinkage is done abruptly to only a section of the media as opposed to the entire sheet at once, shrink cockle results. This can result in printhead crashes at raised regions.
Some types of print media heating techniques assigned to the common assignee of the present invention provide such exemplary prior art solutions:    U.S. Pat. No. 5,287,123 for a PRE-HEAT ROLLER FOR THERMAL INK-JET PRINTER,    U.S. Pat. No. 5,329,295 for a PRINT ZONE HEATER SCREEN FOR THERMAL INK-JET PRINTER,    U.S. Pat. No. 5,399,039 for an INK-JET PRINTER WITH PRECISE PRINT ZONE MEDIA CONTROL,    U.S. Pat. No. 5,406,321 for a PAPER PRECONDITIONING HEATER FOR INK-JET PRINTER,    U.S. Pat. No. 5,428,384 for a HEATER BLOWER SYSTEM IN A COLOR INK-JET PRINTER,    U.S. Pat. No. 5,461,408 for a DUAL FEED PAPER PATH FOR INK-JET PRINTER,    U.S. Pat. No. 5,467,119 for an INK-JET PRINTER WITH PRINT HEATER HAVING VARIABLE HEAT ENERGY FOR DIFFERENT MEDIA,    U.S. Pat. No. 5,510,822 for an INK-JET PRINTER WITH HEATED PRINT-ZONE, and    U.S. Pat. No. 5,668,584 for a METHOD OF MULTIPLE ZONE HEATING OF INKJET MEDIA USING (A) SCREEN PLATEN.
In U.S. Pat. No. 5,742,315, Szlucha et al. describe a SEGMENTED FLEXIBLE HEATER FOR DRYING A PRINT IMAGE. A segmented flexible heater is disposed adjacently to a paper path for heating before and during printing. In U.S. Pat. No. 5,896,154 for an INK JET PRINTER, Mitani et al. describe a prior art belt type preheating unit.
In vacuum belt paper transport subsystems, sometimes heat is applied to the main belt with the vacuum being used to ensure contact to a heater. During heating, the paper 16 is dried. As moisture leaves the paper 16, the paper shrinks. This shrinkage is a change in paper size that is not matched by an equivalent change in the belt 32. Therefore, there will generally be relative motion between the two when the shrinkage occurs as the paper 16 is being transported by the belt 32 which can lead to dot placement error.
In vacuum belt systems, “edge-scalloping” of the sheet is a common occurrence. Edge-scalloping is generally a waviness occurring along the edges of a sheet due to a difference in the drying time from the central regions of the sheet, another form of cockling as described above. Edge-scalloping is a result of cockling effects compounded by irregular drying across the page area. The difference in heat exchange between the heater and the sheet is exacerbated in a vacuum transport system because vacuum loss around the sheet edges can lead to a loss of contact with a resultant loss of heat transfer. The interior regions of the sheet can dry faster and shrink faster than the edge regions. The resultant distortion is scalloped edges.
Actual shrinkage and other shape changes will of course be dependent on actual moisture content and paper thickness. Thus, preheating and print-zone heating of the paper can affect ultimate print quality characteristics. Temperature control is yet another factor which will be dependent on throughput time and media type.
There is a need for improved techniques of print media heating and flattening for a vacuum belt hard copy apparatus.